Fly-by-night and Insubstantial

Fly-by-night adjective - (of businesses and businessmen) unscrupulous.

Insubstantial and fly-by-night are semantically related In some cases you can use "Insubstantial" instead an adjective phrase "Fly-by-night".

Insubstantial adjective - Being of a material lacking in sturdiness or substance.

Fly-by-night and insubstantial are semantically related in weak topic. Sometimes you can use "Fly-by-night" instead an adjective "Insubstantial".